The present invention is directed to a positioning device for a selection member of printing machines. The positioning device comprises a motor and a control circuit which carry out on said selection member displacements which are multiples of an elementary step. For instance, the displacements necessary to rotate a type carrier unit for positioning a type in front of a writing point, or displacements necessary for a carriage acting as support of the type carrier unit, may be carried out by the positioning device.
In some known devices, the positioning of these selection members is obtained through closed loop or open loop servomechanisms. Said mechanisms usually utilize a direct current motor controlled by feed back signals of position and/or speed transducers, or a stepping motor that can avoid the use of transducers. Both solutions are expensive. In one case, expense is incident to the high precision required for the transducer, which identifies the exact position and the speed of the rotor, in case of direct current motors. In the other case, expense is incident to the high intrinsic cost of stepping motors that should be utilized for the same purpose.
A first object of the present invention is to realize a positioning device for a selection member of printing machines which is exact, of low cost and which performs fast writing cycles.
Servomechanisms that discriminate only a coarse area near the desired position requested by the selection member have also been used. To these servomechanisms are coupled mechanical positioning devices, in order to correct the reached approximate position. Although the positioning devices are rather cheap, the cost of these realizations is still not acceptable in cheaper printing machines, due to the presence of all those mechanisms necessary to the operation of the mechanical positioner. Besides, these devices are not suitable for high speed printers owing to inertial delays associated with the mechanical positioner.
These disadvantages are solved by the positioning device of the invention, which comprises a coarse positioning detector for detecting elementary displacements of the selection member and a servomotor responsive to the information of the positioning detector to drive the selection member to a coarse position adjacent the requested position. The device further comprises a magnetic positioner having a movable part connected with the selection member and a control circuit for the magnetic positioner actuatable for putting said movable part into a position corresponding exactly to the requested position of said selection member.
The positioning device can be utilized for positioning a rotating member moved by a motor and connected with a piston transducer, including a rotating support having a series of position indicators equally spaced around the periphery of the support and a position detector for detecting the passage of the position indicators in front of a reference index. The signals of the position detectors are sent to an incremental counter to identify the angular position of the rotating member.
In known devices, on a rotating member associated with an incremental position counter, there is generally provided a synchronization detector that resets the content of the position counter when the detector detects the movement of a synchronization index in front of a reading station. The use of the second detector represents an additive cost and reduces the total reliability of the positioning device.
Another object of the invention is to reduce the cost of the means required to synchronize the content of a position counter with the position of a member to be positioned.
According to a first solution, the positioning device comprises a position indicator assymmetric as referred to the equally spaced indicators, a circuit that actuates the motor to rotate the rotating member according to a predetermined velocity, a time detector for detecting the times corresponding to the passage of the position indicators in front of the position detector, and means which load the position counter with a reference value when the time detector detects the passage of the assymmetric indicator, anomalous as referred to passages of equally spaced indicators.
According to a second solution, the positioning device is used on a printer including a type carrier unit of wheel or disk type and a carriage movable in front of the platen, a rotating selection member fitted on a carriage to select a character of said unit and a couple of counters, respectively for selection and transport, whose contents indicate the position of the selection member and the position of the carriage. The synchronization between selection counter and position of the selection member and between transport counter and position of the carriage is realized by a rotary index synchronous with the selection member and a translating index fitted on the carriage, and a single synchronizing transducer for the selection counter and the transport counter. This transducer is fixedly fitted referred to the printing line and is actuatable by the presence of the translating index, when the carriage is on a margin of the printing line, and by the presence of the rotary index, when the carriage is in a predetermined position of extra margin external to the printing line and said selection member is in a predetermined angular position. An initialization circuit moves the carriage towards said margin, independently of the content of the transport counter, till an activation of the synchronization transducer and a consequent synchronization of the transport counter. Successively, the circuit moves the carriage to its position of extra margin under the control of the transport counter and rotates the selection member, till a second actuation of the synchronization transducer and to a consequent synchronization of the selection counter.